Fluidized catalytic cracking (FCC) is an established and widely used process in the petroleum refining industry for converting relatively high boiling products to more valuable lower boiling products including gasoline and middle distillates, such as kerosene, jet fuel and heating oil. The pre-eminent catalytic cracking process is the fluid catalytic cracking process (FCC) wherein a pre-heated feed contacts a hot cracking catalyst. During the cracking reactions, coke and hydrocarbons deposit on the catalyst particles, resulting in a loss of catalytic activity and selectivity. The coked catalyst particles, and associated hydrocarbon material, are stripped, usually with steam, to remove as much of the hydrocarbon material as technically and economically feasible. The stripped particles, containing non-strippable coke, pass from the stripper and to a regenerator. In the regenerator, the coked catalyst particles are regenerated by contacting them with air, or a mixture of air and oxygen, at elevated temperatures, resulting in the combustion of the coke—an exothermic reaction. The coke combustion removes the coke and heats the catalyst to the temperatures appropriate for the endothermic cracking reactions.
The process occurs in an integrated unit comprising the cracking reactor, the stripper, the regenerator, and the appropriate ancillary equipment. The catalyst is continuously circulated from the reactor or reaction zone, to the stripper and then to the regenerator and back to the reactor. The circulation rate is typically adjusted relative to the feed rate of the oil to maintain a heat balanced operation in which the heat produced in the regenerator is sufficient for maintaining the cracking reaction with the circulating, regenerated catalyst being used as the heat transfer medium.
To provide optimal catalytic cracking conditions, one or more nozzles preferably collectively spray the hydrocarbon stream in a pattern that expands to cover substantially the entire cross-sectional area through which the cracking catalyst is flowing. Improved spray coverage provides better catalyst-hydrocarbon feed mixing which enhances catalytic cracking reactions and minimizes thermal cracking reactions. Thermal cracking reactions produce undesirable products such as methane and ethane and decreased yields of more valuable FCC products.
The nozzles preferably produce fine hydrocarbon feed droplets. As droplet size decreases, the ratio of hydrocarbon feed droplet surface area to volume increases, which accelerates heat transfer from the catalyst to the hydrocarbon feed and shortens hydrocarbon feed vaporization time. Quicker vaporization improves yield of catalytic cracking reaction products because the vaporized hydrocarbon feed diffuses into the pores of the catalyst. Conversely, any delay in vaporizing the hydrocarbon feed vaporization and mixing it with the catalyst increases yields of thermal cracking products and coke. Thus, processes and apparatuses that can economically reduce feed droplet size can improve yields in FCC processes.
Single-stage addition of injection or dispersion steam to hot oil for FCC feed injection is well-known in the art. Steam creates a two-phase mixture with oil which promotes formation of liquid ligaments as this oil and steam mixture is ejected through the throat (orifice) of the injection nozzle. These ligaments rapidly break up into smaller diameter droplets. Increasing the kinetic energy of the oil and steam mixture and effectively converting kinetic energy to surface tension energy is believed to improve atomization quality by creation of smaller mean liquid droplet diameters. Methodology for steam addition varies widely. In some instances, steam is simply added via a nozzle or mixing tee connected to the oil feed line upstream of the nozzle. The prior methods seek to obtain a nearly homogeneous mixture of steam and oil upstream of the atomizing nozzle tip. However, the prior methodology does not recognize the ability to achieve even better atomization by combining multiple steam (or other atomization fluid) addition devices as disclosed herein.